This invention relates generally to radiation therapy equipment for the treatment of tumors or the like and specifically to a computerized method for rapidly correcting a radiation treatment plan to account for motion or change in shape of treatment areas.
Medical equipment for radiation therapy treats tumorous tissues with high energy radiation. Such radiation may be x-ray radiation or accelerated electrons, protons, neutrons or heavy ions. The amount of radiation and its placement must be accurately controlled to ensure both that the tumor receives sufficient radiation to be destroyed and that the damage to the surrounding non-tumorous tissue is minimized.
One highly accurate method of controlling the dose to a patient employs a radiation source that produces many individual rays whose intensity and/or energy may be independently controlled. This may be done by a series of shutters each controlling one ray or by a single modulated ray moved across the patient. The origin of the rays orbits the patient within a plane of the rays to illuminate a slice of the patient, when the orbit is planar, or several slices of the patient, when the orbit is helical. By properly selecting the ray intensities and/or energies at different angles, complex regions within the slice may be accurately irradiated. A mapping of the modulation of each beam as a function of angle forms a xe2x80x9ctreatment sinogramxe2x80x9d.
U.S. Pat. No. 5,317,616 issued May 31, 1994 assigned to the same assignee as the present application and hereby incorporated by reference describes the construction of one such machine and a method of calculating the necessary beam intensities and/or energies as a function of angle.
In order to take advantage of the improved accuracy in dose placement offered by such radiation therapy systems, the radiation treatment plan may be based on a computed tomography (CT) image of the patient. As is known in the art, a CT image is produced by a mathematical reconstruction of many projection images obtained at different angles about the patient. In a typical fan beam CT acquisition, the origin of the fan beam orbits the patient within a plane of the fan to illuminate a slice of the patient, while the attenuation of each ray of the fan beam is measured as a function of that angle to obtain projections. The geometry of the CT acquisition is thus very similar to the geometry of the radiation therapy.
Each CT projection forms a one-dimensional line image indicating the attenuation of the fan beam by a xe2x80x9cslicexe2x80x9d of the patient. Together these line images at each angle form an xe2x80x9cattenuation sinogramxe2x80x9d which may be reconstructed using well known algorithms such as filtered back projection into two dimensional tomographic images of the slice. The sinographic data, which by itself is unintelligible, is normally no longer used or accessed by the user.
Using the CT image, the radiologist views the tumorous area and determines the beam angles and intensities and/or energies (identified with respect to the tumor image) which will be used to treat the tumor. In an automated system, a computer program selects the beam angle and intensities and/or energies after the physician creates a dose map identifying the tumorous region and upper and lower dose limits for regions of the treatment.
Preparing a treatment plan based on the dose map is a time consuming operation even on current high speed computers. Accordingly, the CT image of the patient is acquired before the time of radiation treatment. As a result, the patient will typically not be in the same position during the radiation treatment as the patient was during the CT imaging. The problem of properly aligning the patient is compounded when the treatment occurs in a number of different sessions over time.
U.S. Pat. No. 5,673,300 assigned to the same assignee as the present invention describes a method of determining patient movement by obtaining a second CT image immediately prior to radiation therapy and comparing the sinogram of that CT image to the sinogram of the original CT image used for radiation treatment planning. This comparison yields an indication of patient movement which may be applied directly to the treatment sinogram used to control the radiation therapy machine. This invention, by recognizing the close analogy between the attenuation sinograms of the CT image and of the treatment sinograms of radiation therapy treatment, greatly simplified detecting and correcting mis-registrations of the patient to the treatment sinogram.
The present inventors have recognized that the above technique of directly modifying the treatment sinogram, by bypassing the time-consuming translation of dose map to treatment sinogram, makes possible real-time correction for patient motion. Such a correction may deduce real-time motion from a concurrent tomographic scan or from well known transducers used for measuring physiological motion. An improved method for correcting xe2x80x9cfan beamxe2x80x9d sinograms facilitates this use of the sinogram directly. The inventors have also recognized that the ability to manipulate sinograms to accommodate motion in the underlying structure, allows for a novel method of generating a treatment sinogram by combining precalculated partial sinograms representing treatments of standard elements of the patient. These standard elements may be moved to match a particular patient""s anatomy and the partial sinograms modified according to the techniques described above. The partial sinograms are then combined and used directly or as a starting base for iterative treatment planning software.
Specifically, then, the present invention provides a method of operating a radiation therapy machine providing a radiation beam of individually intensity and/or energy modulated radiation rays separated along a radiation beam axis, the radiation beam axis positionable at a range of angles about a patient. A treatment sinogram is received providing intensities and/or energies of different rays for a given angle of the radiation beam, in a row, and intensities and/or energies of a given ray for different angles of the beam axis, in a column, for a patient at a first position. During radiation treatment, data indicating patient movement from the first position to a second position is also received and for each given beam axis angle of the treatment sinogram, the corresponding row of treatment sinogram is altered according to the indicated movement.
Thus it is one object of the invention to make possible real-time correction of patient motion to correct not only patient positioning errors but physiological motions such as caused by respiration and cardiac motion. Direct operation on the treatment sinogram renders such real-time control possible.
The movement may be detected by comparing a planning tomographic image of the patient contemporaneous with the preparation of the treatment sinogram to a monitoring tomographic image of the patient taken during radiation therapy. Alternatively, the patient movement may be determined by a model receiving as an input a physiological signal such as respiration or heart beat or external fiducial marks may be measured.
Thus it is another object of the invention to provide a method of detecting patient motion on a real-time basis in a radiation therapy setting.
The modification of the treatment sinogram may shift corresponding rows of treatment sinogram according to a component of patient motion perpendicular to the given beam axis.
Thus it is another object of the invention to provide an extremely simple operation on the treatment sinogram such as may be performed in real-time.
The modification of the treatment sinogram may scale corresponding rows of treatment sinogram according to a component of patient motion parallel to the given beam axis.
Thus it is another object of the invention to provide a more sophisticated modification of the treatment sinogram addressing the geometry of the highly efficient fan beam radiation therapy machine.
The present invention also contemplates the preparation of a library of partial sinograms, each partial sinogram providing intensities and/or energies of different rays at given angles of the radiation beam axis, in sinogram rows, and intensity and/or energy of given rays for different angles of the beam axis, in sinogram columns, for a patient element in first modes. Sets of representations of patient elements may be arranged in combinations at second modes so as to model a given patient requiring radiation treatment. Changes in the patient elements between the first and second modes may be captured in alteration data. This alteration data may be used to modify the partial sinograms of each of the patient elements according to the alteration data and the partial sinograms may be combined to produce a treatment sinogram of the patient.
Thus it is another object of the invention to make use of the ability to directly modify treatment sinograms to prepare template sinograms that may be simply combined to produce a treatment sinogram without the need for extensive treatment planning operations. The alteration data may indicate either change in location or dimension of the patient elements, the latter which may be simple geometric shapes or may model specific organs.
Thus it is another object of the invention to provide in a finite library of patient elements sufficient to permit assembly of an approximate treatment sinogram.
The treatment sinogram thus constructed may be further optimized to better conform with the dose map.
Thus it is another object of the invention to provide an advanced starting point for dose optimization such as may reduce the number of iterations and thus the time required to prepare the treatment sinogram.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessary represent the full scope of the invention, however, and reference must be made to the claims herein for interpreting the scope of the invention.